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Texture-Dependent Adhesion in Polydopamine Nanomembranes Ik Soo Kwon,† Guannan Tang,† Po-Ju Chiang,† and Christopher J. Bettinger*,†,‡ †
Department of Materials Science and Engineering and ‡Department of Biomedical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States S Supporting Information *
ABSTRACT: The conformal nature of in situ polymerization of adhesive dopamine molecules permits the strong underwater adhesion between polydopamine (PDA) nanomembranes and the target substrates. However, the adhesive interaction between the postdeposit PDA nanomembranes and other macrobodies is strongly influenced by the texture of PDA nanomembranes. Here we report the texture-dependent adhesion of PDA nanomembranes both in air and aqueous environments. Despite the nanometer-scale roughness of PDA nanomembranes, interfacial adhesion between PDA nanomembranes and elastomeric bodies are the strong function of the root-mean-square roughness of PDA nanomembranes, root-mean-square gradient of PDA nanomembranes, and the elasticity of the bulk materials. Reduced adhesion due to increased texture is intensified in hydrated conditions, possibly hinting that the conventional explanation of the negative effect of water to adhesion from a molecular level needs to be revisited. These findings can inform the role of adhesive interaction in conformal coatings and provide an explanation for the differential adhesion observed in freestanding PDA nanomembranes. KEYWORDS: polydopamine, surface roughness, differential adhesion, underwater adhesion, DOPA, catechol
1. INTRODUCTION Catechols exhibit strong molecular underwater adhesion, making materials containing this motif interesting candidates for applications including surgical sealants, underwater adhesives, and functional materials for marine environments.1−5 Interest in the catechol-containing systems was initially motivated by the presence of L-3,4-dihydroxyphenylalanine (DOPA) in byssus threads that enable the mussel to attach onto various surfaces underwater.6 Various synthetic polymers with catechol moieties have been subsequently designed, synthesized, characterized, and processed into various formats including adhesive hydrogels,7 bulk films8 and low-molecular peptides.9 Among various catechol-containing polymeric systems, mussel-chemistry-inspired polydopamine (PDA) has been of particular interest ever since the initial work in 2007 by Messersmith et al.10 PDA nanomembranes formed by facile one step auto-oxidative polymerization of dopamine (3,4-dihydroxyphenylethylamine)11 and their analogues adhere tightly to many substrates including polymers, ceramics, and metals.10 Strong adhesion of PDA in diverse environments is attributed to robust molecular adhesion through catechol−substrate interactions2 and the conformal nature of PDA nanomembranes resulting from in situ polymerization. More recently, PDA nanomembranes prepared on oxides can be delaminated and used as freestanding films.12 This processing capability produces a new parameter space that can govern adhesion in freestanding PDA nanomembranes including texture and mechanical properties of the substratum.13 An understanding of the texture-dependent properties of PDA nanomembranes, © XXXX American Chemical Society
including adhesion, is critical to advancing their use as functional materials. Here we measure texture-dependent adhesion of PDA in various conditions by using microindentation measurements and applying Johnson−Kendall− Roberts (JKR) contact theory.
2. EXPERIMENTAL SECTION 2.1. Freestanding PDA Nanomembranes for Contact Angle Goniometry Preparation. Silicon wafers with 1 μm thermal oxide (100 mm diameter, p-doped Si, Silicon Quest International, San Jose, CA, U.S.A.) were cleaned by sonication in acetone, followed by isopropyl alcohol and ddH2O. Substrates were then treated with UVozone (5 min, 30 mW cm−2, Jelight, Irvine, CA, U.S.A.). PDA nanomembranes were prepared by incubating substrates in 2 mg/mL dopamine hydrochloride in 100 mL of 10 mM Tris buffer (Fisher Scientific, Hampton, NH, U.S.A.) in ambient air and orbital rotation (65 rpm). After 24 h, the substrates were rinsed with ddH2O and placed in 200 mM NaCl + 50 mM Tris buffer solution for >6 h for film delamination. Freestanding delaminated nanomembranes were then transferred to ddH2O prior to subsequent processing. 2.2. Transfer Printing of PDA Nanomembranes to Silicon Wafers. Freestanding PDA nanomembranes were placed in ddH2O along with precleaned silicon wafers in Petri dishes. PDA nanomembranes were positioned atop silicon substrates with either the Special Issue: 10 Years of Polydopamine: Current Status and Future Directions Received: October 13, 2017 Accepted: January 22, 2018
A
DOI: 10.1021/acsami.7b15608 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 1. (a) Schematic of freestanding PDA nanomembrane preparation procedure. Incubation of PDA prepared on SiO2 in a weakly basic electrolyte results in the delamination of PDA nanomembranes. (b) Schematic of preparation procedure and AFM images of apical/basal side PDA samples. AFM scan size is 50 × 50 μm2. The surface originally facing the substrate before PDA delamination is termed the basal surface, while the opposite is termed apical. The morphology of apical surfaces is controlled through PDA deposition times and sonication post-treatments. (c) Owens−Wendt plot for basal/apical surfaces (see Supporting Information). Both surfaces exhibit comparable values for interfacial surface energy. Values for the (d) RMS roughness (hrms,PDA) and (e) RMS gradient (h′rms,PDA) of PDA nanomembranes as a function of synthesis conditions and post-treatment. repeated to prepare films with the desired roughness (24, 36, 48, or 72 h). Some samples were placed in 50 mM Tris HCl solution (pH = 8.5) and sonicated for 2 h. For the basal side sample fabrication, epoxy (Aqua Marine Epoxy, Loctite, Düsseldorf, Germany) was deposited on the PDA nanomembrane on a silicon wafer (24 h deposition; no sonication), and a glass slide was placed on top of it. Epoxy−PDA− silicon stacks were placed in 200 mM NaCl + 50 mM Tris buffer solution at 50 °C for 4 h, and epoxy−PDA was separated while in the buffer solution, and the sample was immersed in ddH2O for >24 h prior to performing adhesion measurements. 2.5. Morphological Characterization of PDA Samples. PDA nanomembrane roughnesses were measured using atomic force microscopy (NTegraAFM, NT-MDT, Tempe, AZ, U.S.A.) in tapping mode. The scan areas were 50 × 50 μm2 and recorded at 0.5 Hz using tips with a reported radius of 20 nm underwater. software (http://gwyddion.net/) and a surface topography analyzer (http://contact.engineering/). 2.6. PDMS and Ecoflex Lens Fabrication. Molds for elastomeric lenses were created by using 6 mm diameter borosilicate glass hemispherical lenses (Edmund Optics, Barrington, NJ, U.S.A.) and Norland Optical Adhesive 81 UV Glue (Norland Product, Cranbury, NJ, U.S.A.). Fabricated molds were immersed in a solution made by combining 10 mL of trichloroethylene and 12 μL of (tridecafluoro1,1,2,2-tetrahydroctyl)-trichlorosilane (Gelest, Morrisville, PA, U.S.A.) for 16 h, followed by sonication in isopropyl alcohol and then in ddH2O and drying under an N2 stream. The Ecoflex precursor was made by mixing part A and part B of Ecoflex 00-30 (Smooth-On, Macungie, PA, U.S.A.) at a 1:1 ratio, degassing ( 0.995), which indicates that bulk deformation is largely elastic.24 We speculate that hysteresis between loading and unloading phases is attributed to interfacial dissipation, behavior that is qualitatively similar to that described in previous reports for PDMS lenses.23−26 JKR theory suggests that the maximum force or pull-off force during unloading (Fmax) in a perfectly elastic response with the absence of surface roughness is related to the thermodynamic work of adhesion by
(1)
1
6πRGF + (3πRG)2 ]
3R [F 4E *
−
contact area radius a and indentation depth δ are given as a function of the work of adhesion (energy release rate between two surfaces; G) 3R [F + 3πRG + 4E*
2πaG E*
The combination of eqs 2 and 3 yields the following relationship for indentation depth
where, ν1 and ν2 are Poisson’s ratios of the hemispherical lens and PDA nanomembranes. Since E2 ≫ E1, the combined E modulus can be expressed as E* ≈ 1 − 1ν 2 . Expressions for the
a3 =
a2 − R
(2) D
DOI: 10.1021/acsami.7b15608 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces F
Gthermo = 1.5max .20 Even in cases where interfacial dissipation is πR non-negligible and surface roughness is present, the same
expression
(
Fmax 1.5πR
) gives the effective work of adhesion (W
UL),
a
key figure of merit in measuring texture-dependent adhesion,27 as long as the following conditions are met: the dimensions of the surface roughness are small compared to the overall deformation of the hemispherical lenses. For measurements between PDMS lenses and PDA nanomembranes with hrms,PDA > 60 nm in air, force−displacement curves in the loading phase do not permit the fit to eq 5, which we attribute to the higher roughness of these samples (see Supporting Information). For measurements between Ecoflex lenses and PDA nanomembranes, regardless of the hrms,PDA of PDA nanomembranes, the force−displacement curves during the loading phase show a good fit to eq 5 with a G value similar to that of PDMS (slightly smaller) and show a similar degree of hysteresis (see Supporting Information). 3.4. Force−Displacement Measurements in Water. A “jump-in” is a surface free energy induced mechanical instability transition that often occurs while conducting adhesion measurements, meaning two surfaces jump into each other, establishing a finite contact even at zero load.28 Jump-in was observed between PDA nanomembranes and both PDMS and Ecoflex lenses in air. However, these artifacts were not observed in aqueous environments.14 Eq 5 could be fit to the loading phase of force−displacement curves for both PDMS and Ecoflex lenses underwater. However, the value of G was near zero throughout the loading phase for both PDMS and Ecoflex lenses (Figure 2c) (see Supporting Information). Also, the measurements against PDA nanomembranes with hrms,PDA greater than 20 nm largely abolish the adhesion (pull-off force) for measurements with both PDMS and Ecoflex lenses. 3.5. Effect of Roughness on Adhesion. From the pull-off forces from indentation measurements, plots of WUL vs RMS roughness (hrms,PDA) of PDA nanomembranes and WUL vs RMS gradient (h′rms,PDA) of PDA nanomembranes were constructed (Figure 3). Figure 3a shows the plot of WUL vs hrms,PDA in air. For PDA nanomembranes of hrms,PDA < 60 nm, WUL from measurements with PDMS and Ecoflex lenses did not show the trend of WUL value decreasing with increasing hrms,PDA. For Ecoflex lenses, the same trend (WUL not decreasing with increasing hrms,PDA) held true throughout the hrms,PDA range explored ( 60 nm. The observed behavior is in reasonable agreement with theoretical predictions (Figure 3a) (see Supporting Information). The slight overestimation in WUL is anticipated, as the surface estimation through Gaussian distribution underestimates the effect of outliers (see Supporting Information).29,30 Experimental data follows the key characteristic features expected from the theory: (1) there is an elasticity-dependent reduction in the work of adhesion; (2) the work of adhesion is nearly constant for roughness values smaller than a certain threshold value (hrms,PDA = 60 nm for PDMS and hrms,PDA > 200 nm for Ecoflex), which is also modulus-dependent. WUL can also be plotted against another roughness parameter, RMS gradient (h′rms,PDA) (Figure 3b). WUL and h′rms,PDA are inversely correlated with similar key characteristic behaviors observed in the plot of WUL vs hrms,PDA. Namely, the value of WUL between PDMS and PDA decreases with increasing h′rms,PDA for h′rms,PDA values beyond a threshold value. Also, the value of
Figure 3. (a) Plot of the effective work of adhesion vs RMS roughness of PDA nanomembranes (WUL vs hrms,PDA) in air. (b) Plot of the effective work of adhesion vs RMS gradient of PDA nanomembranes (WUL vs h′rms,PDA) in air. (c) Plot of the effective work of adhesion vs RMS roughness of PDA nanomembranes underwater (WUL vs hrms,PDA). (d) Plot of the effective work of adhesion vs RMS gradient of PDA nanomembranes underwater (WUL vs h′rms,PDA). Highlighted regions contain points for basal PDA surfaces and apical PDA surfaces with 24 h deposition and subsequent sonication (c,d). Data plotted as mean ± s.d. for n = 3.
WUL between Ecoflex and PDA is nearly constant for the ′rms,PDA range explored in this study. Figure 3c,d shows the plots of WUL vs hrms,PDA and WUL vs h′rms,PDA underwater. As noted previously, nonzero adhesion (pull-off force) was observed only for hrms,PDA < 20 nm for both PDMS and Ecoflex lenses (Figure 3c). Interestingly, values for WUL for basal PDA surfaces (hrms,PDA = 18.1 nm) are larger than that of apical PDA surfaces with 24 h sonication (hrms,PDA = 8.3 nm) despite the former having a larger hrms,PDA than the latter (Figure 3c). For this situation, it could be instructive to examine another roughness related parameter; RMS gradient. Basal PDA surfaces exhibit a lower RMS gradient than the apical surface of PDA membranes that formed through 24 h deposition and sonication (h′rms,PDA = 0.047 vs h′rms,PDA = 0.16). Abolished adhesion underwater presents a departure from thermodynamic calculations, which predict a larger work of adhesion between PDMS and PDA for the underwater conditions compared to air (see Supporting Information). To our best knowledge, no satisfactory model currently exists that can model the current experimental condition reasonably. We cautiously speculate that a different kind of texture-dependent mechanism than in air environments is at play in aqueous environments. Furthermore, the underwater contact mechanics could be more sensitive to an RMS gradient compared to an RMS roughness, as the many contact properties are largely dominated by one of the surface roughness related parameters.31 For example, RMS gradient and RMS roughness are more relevant in surfaces with smaller and larger characteristic features, respectively. The monotonic decrease in WUL with increase in h′rms,PDA observed in WUL vs h′rms,PDA plot is in agreement with the assertion (Figure 3d). However, additional E
DOI: 10.1021/acsami.7b15608 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 4. Schematic of contact between PDA nanomembranes on substrates interfacing with elastomeric spherical probes. (a) The contact region between the lens and PDA substrates is magnified in subsequent panels. (b-i) Ecoflex conforms well to interfaces composed of PDA nanomembranes with both hrms,PDA > 60 nm and hrms,PDA < 60 nm. (b-ii) PDMS conforms poorly to PDA interfaces with hrms,PDA > 60 nm. (c) Aqueous environments convolve the effect of surface roughness, translating to lower contact area once the water layers infiltrate the lens/substrate interface.
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study is needed to draw a conclusion regarding the role of RMS gradient in underwater adhesion for PDA nanomembranes. Nevertheless, the observed trend signifies the importance of morphological control in underwater adhesion. The proposed physical model to explain texture-dependent adhesion in various environments is shown in Figure 4. In air, both Ecoflex (low elasticity) and PDMS (high elasticity) lenses conform well to smooth PDA surfaces (low hrms,PDA). For rough PDA surfaces (high hrms,PDA), Ecoflex lenses still conform well, attested by the negligible WUL change with respect to change in hrms,PDA. PDMS lenses conform poorly to PDA surfaces with high hrms,PDA, translating to the lower work of adhesion. For the underwater case, the effect of roughness intensifies, with adhesion completely removed for PDA surfaces with hrms,PDA > 20 nm. Also, the observed trend hints that adhesion reduction underwater shows a stronger dependency on h′ rms,PDA compared to hrms,PDA even though the role of mechanics in the probe is unclear. The finding suggests that the suppression of adhesion underwater, often explained exclusively from a molecular level, might need to be revisited by carefully considering the morphology of the samples with special focus on controlling RMS gradient of surfaces.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b15608. Owens−Wendt plot construction; Young’s moduli comparison between tensile and indentation experiments; representative force−displacement curves for other experimental conditions; computation of the theoretical curve for the effective work of adhesion versus RMS roughness in air (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ik Soo Kwon: 0000-0002-2835-5511 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank Dr. Luke Klosterman for helpful discussions. The authors acknowledge financial support provided by the following organizations: National Institutes of Health (R21NS095250); the Defense Advanced Research Projects Agency (D14AP00040); the National Science Foundation (DMR1542196); the Carnegie Mellon University School of Engineering; the authors would also like to thank the CMU Thermomechanical Characterization Facility in the Department of Materials Science and Engineering and acknowledge the Materials Characterization Facility at Carnegie Mellon University supported by Grant No. MCF-677785.
4. CONCLUSION The role of texture in adhesive interaction of PDA nanomembranes was studied in various contexts. Microindentation tests elucidated the complex relationships between PDA texture, the elasticity of interacting bodies, and interfacial adhesion in both in air and underwater conditions. The nanometer-scale random roughness inevitably introduced to PDA nanomembranes during synthesis can significantly alter the adhesive behavior of PDA nanomembranes, and the fact suggests that the adhesion of PDA nanomembranes can be tuned through morphology control. Our findings suggest that the texture-dependent adhesion could explain differential adhesion of PDA to many classes of materials. This study also highlights the possible importance of designing interfaces with low RMS gradients to ensure underwater adhesion. This trend, observed in the context of PDA nanomembranes, could be extended to other types of materials as well. Generalizable trends can inform the design of PDA-based adhesives or other catechol-bearing functional materials that are designed to operate in aqueous conditions.
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DOI: 10.1021/acsami.7b15608 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
